The present invention is directed to a highway crash cushion such as an energy absorbing support frame for a vehicle mounted attenuator.
Vehicle mounted attenuators (so called truck mounted attenuators or TMAs) are currently in widespread use. U.S. Pat. Nos. 4,711,481 and 5,199,755, assigned to the assignee of this invention, describe several such TMAs. In each case, the TMA is designed to be mounted on a vehicle such as a truck, which is then positioned adjacent to a region to be protected, such as a work zone alongside a highway. A vehicle which leaves an adjacent lane of traffic will be prevented from entering the work zone when it hits the TMA. The TMA decelerates the impacting vehicle in a controlled manner and reduces damage to the vehicle and injury to the occupants of the vehicle.
One problem in TMA design is that the device should operate reliably both in stopping lightweight vehicles travelling at relatively slow speeds (a low-energy event) and heavy vehicles travelling at high speeds (a high-energy event). In order to expand the energy absorbing range of TMAs, support frames of various types have been developed, including those described in U.S. Pat. Nos. 5,248,129 and 5,403,113, assigned to the assignee of this invention. These support frames are interposed between the TMA and the vehicle, and they provide an additional level of energy absorption capacity for high-energy events.
The support frame shown in U.S. Pat. No. 5,248,129 has been found to operate successfully in actual use. However, this support frame suffers from two disadvantages. First, the support frame is arranged about horizontal hinge axes, and after the support frame begins to collapse it provides little or no vertical support to the TMA. This is generally not a problem, because the impact occurs over such a short period of time that the TMA does not have time to move vertically in a disadvantageous manner. Nevertheless, this arrangement of the hinge axes leaves the TMA vulnerable to damage, in the event the support frame inadvertently collapses due to a failure of the restraining cables.
Second, the support frame shown in U.S. Pat. No. 5,248,129 relies on breakaway fasteners to hold the support frame in the initial configuration. When the compressive loads on the support frame exceed a predetermined level, these breakaway fasteners part, thereby allowing the support frame to begin to collapse. It can be expensive to obtain and maintain breakaway fasteners that operate with the desired degree of precision and reliability. Furthermore, a breakaway system that responds to the magnitude of the collapsing force may in some cases allow the support frame to collapse prematurely, in a low-energy event. In this case maintenance costs for the support frame can be unnecessarily high.
The present invention is directed to improvements which substantially overcome the disadvantages discussed above.